Along with the advance in technology, the demand for high-energy electronic element is ever increasing. A conventional Si-based and gallium arsenide (GaAs) element has a small energy gap, and therefore fails to comply with the application requirements of high-energy element and must change to a semiconductor material having a wide energy gap such as silicon carbide (SIC) or a III-nitride based nitride such as gallium nitride (GaN). For example, the high electron mobility transistor (HEMT) has higher channel electron mobility and carrier concentration, and better meets the application requirements of high-energy electronics.
The III-nitride based semiconductor structure (such as III-nitride based HEMT) has strong polarization and piezoelectric effects, and will therefore generate two-dimensional electron gas (2DEG) having high density of carriers. The two-dimensional electron gas refers to the electron gas which can move free in two dimensions but is restricted in the third dimension. The two-dimensional electron gas significantly increases the mobility rate of the carriers/electrons of a transistor. However, the two-dimensional electron gas makes the normally-off operation more difficult. According to a conventional method for resolving the above problem, the gate electrode is recessed using a plasma etching process or a p-type GaN layer is added to the underneath of the gate electrode. However, the plasma etching process may easily damage the surface of the structure layer and jeopardize the electronic properties of the element. Moreover, adding a p-type GaN layer to the underneath of the gate electrode will increase the distance between the two-dimensional electron gas and the gate electrode and deteriorate the transconductance (gm) of the elements.